This invention relates to mixer circuits for use in communications devices and the like.
More particularly, the present invention relates to an improved and greatly simplified mixer circuit which is useful, for example, in converting input radio frequency signals into intermediate frequency signals.
Generally, communications devices, such as radios, wireless telephones, televisions, etc. incorporate a front end that receives transmitted radio frequency (RF) signals and converts the RF signals into intermediate frequency (IF) signals that are lower in frequency and, thus, easier to work with. Generally the front end includes some amplification of the received RF signals, after which the RF signals are applied to one input of a mixer circuit. A local oscillator is connected to a second input of the mixer circuit. The mixer circuit then mixes or combines the RF signals with the local oscillator signal and produces output signals consisting RF frequency f1, local oscillator frequency f2, difference signal f2xe2x88x92f1, sum signal f2+f1, and other higher order products. A filter circuit is incorporated at the output terminal of the mixer to select the desired frequency signal and to reject other signals. A mixer circuit that produces the difference signal is generally known as a down converting mixer. A mixer circuit that produces the sum signal is generally is known as an up converting mixer. A down converter mixer is typically used in the receiver section of a communication device while an up converting mixer is utilized in the transmitter section of a two way communication device such a cell phone or wireless telephone. This difference or sum signal is referred to as the IF signal.
All known prior art circuits require a separate mixer and a separate controllable local oscillator, such as a voltage controlled oscillator. The local oscillator is then coupled to the mixer circuit by some matching circuit. The separate local oscillator, mixer, and matching circuitry require relatively large amounts of space. Because the mixer and local oscillator are separate circuits, the matching circuitry often requires discrete components (e.g., capacitors and/or inductors) which are difficult to provide and position in the circuit. Also, because the mixer and local oscillator are separate and require connecting and matching circuits, substantial loss occurs in the various signals. Further, in many instances some form of shielding or radiation separation/positioning must be used, to remove any possibility of interaction, other than the desired mixing.
Accordingly it is highly desirable to provide apparatus which overcomes the above described problems
It is an object of the present invention to provide new and improved mixer circuitry which is much simpler and which requires substantially smaller space and fewer components.